The present invention relates to the technical field of radiology, and especially to beam limiting units used to define the shape of the X-ray beam emitted by an X-ray tube during the acquisition of a radiography.
The X-ray tube generally emits an X-ray beam, which is limited by beam stops near the X-ray source to a generally conical or pyramidal form. The beam is then further modified to obtain a final shape, which is defined downstream the emission point by guiding the X-ray beam through an aperture allowing it to get the desired shape and dimensions with greater precision. Usually with the passage through the aperture the X-ray beam assumes a square or rectangular shape.
In the different types of radiological acquisition, apertures having different shapes and dimensions must be interposed between the X-ray source and the X-ray detector, with the aim of conferring to the X-ray beam the shape and the dimensions optimal for that particular acquisition.
The use of beam limiting units is well known in the art, and in particular the use of beam limiting units which use one or more blades which can be moved in different ways through suitable actuators. These beam limiting units sometimes also form part of a collimator.
Typically a beam limiting unit is made of two blades and one or two actuators moving the blades. In this way, an aperture having a fixed height (corresponding to the blades height) and variable width (the distance between the internal edge of the two blades) is obtained. Superimposing perpendicularly two beam limiting units, an aperture having variable height and width can be obtained. The superimposition of two beam limiting units is known in the art.
The central rays of the final X-ray beam that has passed through the aperture must be in the desired position. One of the limits of prior art is the impossibility of setting the central ray of the final X-ray beam where desired, due to limitations linked to blades position. For instance, in the beam limiting unit known from DE 32 36 082 A1, the two blades move in a symmetrical way with respect to the central ray, which, once fixed, cannot be moved.